A multiple wavelength signal generation device can be used as a light source for a wavelength multiplexed transmission system, a reference light source for measurement, and the like. Therefore, various types of multiple wavelength signal generation devices have been proposed. As specific multiple wavelength signal generation devices, one having integrated numerous semiconductor lasers (LD), a super continuum light source utilizing non-linearity of a fiber, a mode-locked laser using a fiber ring, and an optical comb generator using a Fabry-Perot optical modulator are known.
For the mode-locked laser and the Fabry-Perot optical modulator, phase relationships between wavelength contents are determined, and wavelength intervals are accurately constant. However, a multiple wavelength signal generation device using them is required to stabilize an optical path, so that the apparatus becomes complicated. Also, since the super continuum light source uses the mode-locked laser, there is a similar problem as that of the mode-locked laser.
In case an optical comb generator is used as the multiple wavelength signal generation device, phase relationships between wavelength components need not be determined. Accordingly, an optical comb generator using an optical SSB modulator instead of a Fabry-Perot optical modulator has been developed. (see [T. Kawanishi, S. Oikawa, K. Higuma and M. Izutsu, “Electrically Tunable Delay Line Using an Optical Single-Side-Band Modulator” IEEE Photon. Tech. Lett., Vol. 14, No. 10, 1454-1456 (2002)], [Tetsuya Kawanishi, Masayuki Izutsu, “Optical Comb Generation Using a SSB Modulation Optical Loop And Variable Optical Delay Line” Shingaku Giho (Technical Report of IEICE) 2004-04, pp. 13-18 (2004)]).
FIG. 4 shows a basic arrangement of a conventional optical comb generator using an optical SSB modulator (hereinafter, also called simply as “optical comb generator”). As shown in FIG. 4, an optical comb generator (100) is composed of an optical fiber loop (105) provided with an optical SSB modulator (101), an optical amplifier (102) for compensating a conversion loss by the optical SSB modulator, an optical input port (103), and an optical output port (104). It is to be noted that the optical SSB modulator is an optical modulator capable of obtaining an output light having shifted just an amount of a frequency of a modulating signal. Hereinafter, a basic operation of the optical comb generator will be described.
An input light (106) is inputted to the input port (104) of the optical comb generator. The input light is a continuous light (f0) of a single mode. Then, a frequency of the input light is shifted (f0+fm) by the optical SSB modulator (101). A light component (107) whose frequency has been shifted, circles the loop to be combined with a new light inputted to the input port (f0, f0+fm). These lights are guided to the optical SSB modulator (101), and frequencies of both components are shifted (f0+fm, f0+2fm). By repeating these processes, lights having numerous spectrum components (an optical comb) can be obtained. While phase relationships between wavelength components are unstable, since wavelength intervals are constant with a good accuracy and there is little necessity to stably control an optical path, the optical comb generator has an advantage that a simple apparatus is adequate.
Thus, with an optical comb generator, a plurality of wavelength components is included in a loop. Therefore, in order to prevent a light interference within the loop, a single light unmodulated has been used as the input light.
When a device for a wavelength multiplexed optical transmission system is tested by using multiple wavelength lights generated with such an optical comb generator, the testing has been performed by collectively modulating the multiple wavelength lights and by superimposing signals (for example, FIG. 2 of [L. D. Garrett, A. H. Gnauck, Member, IEEE, F. Forghieri, V. Gusmeroli, and D. Scarano, “16×10 Gb/s WDM Transmission Over 840-km SMF Using Eleven Broad-Band Chirped Fiber Gratings”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 4, APRIL 1999]). When the device for the wavelength multiplexed optical transmission system is tested by using such multiple wavelength lights, since the test has been performed by using the lights having the same optical phase or the like, it cannot be deemed that the test is performed appropriately. Also, in FIG. 1 of [Hiro Suzuki, Jun-Ichi Kani, Hiroji Masuda, Noboru Takachio, Katsumi Iwatsuki, Yasuhiko Tada, and Masatoyo Sumida, “1-Tb/s (100 10 Gb/s) Super-Dense WDM Transmission with 25-GHz Channel Spacing in the Zero-Dispersion Region Employing Distributed Raman Amplification Technology” IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 7, JULY 2000], after having bundled even numbers and odd numbers of wavelength components, intensities are modulated by using signals having time differences, thereby performing an experiment related to the wavelength multiplexed system using such lights. Also in this example, while modulation patterns are different between adjacent channels, the same patterns appear after every other channel. Therefore, as in the above example, it cannot be deemed that a test is performed appropriately. Therefore, a multiple wavelength signal source including more modulations with a simple apparatus have been desired.